Legal protective measures making the copying of keys illegal and practical protective measures making copying very difficult are used against the forging of keys. With respect to the practical measures, a distinction can be made between those bringing about concealment or secrecy and those which make manufacture difficult. In the latter case the manufacture is made so difficult as a result of the mechanical conditions, that only appropriately equipped key copiers or forgers can carry out manufacture. Combinations of these are used in order to provide a practical protection.